of course it's possible, just depends on where the hole lines up and if the devon one is smaller? westy is a fair size hole but anyhing is possible.
Are you wanting to change just the physical roof and not the bed set up? If you want to change the bed then the hole is pretty big and there is a steel crossmember in line (ish) with the back of the Slider on a Westy that I think devon's don't have.
front hingers i think, i'm fairly sure they cut a sizeable chunk out of the rear hinged and weld a new reinforcing section it that the upstairs bed is on?
Mt sister had it done to hers, brand new westy roof fitted and bought from o'conners campers i think, also tezzer on here had it done last year at the same place i think
think it's been mentioned already, but I've a feeling Devon cut further towards the rear end for their conversions, so you'd need to locate a cross member further forward to regain the strength if you want to do the upstairs bed thing. Easy enough to make....couple of pieces of 22mm ply edge on, glued and screwed, with two hangers fabbed-up to hook onto the roof edges...if you get the end profile bob on it will be self supporting as the hangers would be bearing down on the cross-member under load
